Re: 1972 Merc Stopped going Forward
Not familiar with your engine, but Merc changed from a cotter pin shear pin arrangement to a rubber hub prop somewhere back in time. The shear pin arrangement was nothing more than a length of bronze/brass rod that ran through the prop and the prop shaft (to lock them together) and was located under the prop hub cap which was secured with a cotter pin. If the prop struck something hard enough the rod would shear, allowing the prop to freespin and thereby protect the gear train.
Since there were times when you were at risk and had to have drive power, (windward side of rip-rap for example) Merc decided to use a rubber slip hub without the pin and when something was struck hard enough, the hub would slip allowing the prop to stop (when it hit the object) while the drive train continued rotating. Putting the shifter in N took the pressure off the hub, it reseated, and was ready to go back to work immediately. Some hubs are worn and cannot reseat properly and therefore allow the engine to rev without any/little forward or reverse motion of the boat. Time to re-hub or re prop.
Checking for a spun hum is easy. Take a magic marker (permanent ink) and mark the hub drawing a line from the splined part of the hub, straight out to the outer edge of the prop hub.
Put things back and put the boat back in the water and attempt to go somewhere. Then get back on the trailer or whatever you choose and remove the hub cap and look at the mark you made on the prop hub. If it is not still aligned, you have a spun hub and need to take appropriate action.
HTH,
Mark
